This invention relates to a system for correcting a positional error in a numerical control device of the type used to control a machine tool.
When controlling the feed of a workpiece table in a machine tool such as a lathe, positional errors stemming from such mechanical phenomena as pitch error and backlash occur. A numerical control device ordinarily is furnished with pitch error and backlash correction functions that the numerical control device relies upon to correct positional errors when controlling a machine tool. One method of correcting pitch error is disclosed in Japanese Patent Application Laid-Open No. 50-63381 published May 29, 1975, in which the correction is effected by purely electrical means. Specifically, a mechanically movable body has several socalled control axes along which the body can be moved. The body may, for example, be capable of movement along the X and Y axes. In the aforesaid method, the range of movement along each axis is divided into a plurality of segments. For example, the movable range of the body along the X-axis is divided into AX.sub.i segments(where i=1, 2, . . . ), and pitch error correction factors EX.sub.i (i=i, 2, . . . ) are stored in memory so as to correspond to each of said segments. Then, when the movable body is moved along the X-axis so that the current position of the body lies on a prescribed segment EX.sub.i, the pitch error correction factor EX.sub.i corresponding to that segment is read out of the memory, the current position of the body along the X-axis being corrected on the basis of the correction factor EX.sub.i and the direction of movement, i.e., in the plus or minus direction. This method enables the correction along the X-axis to be effected in a highly accurate manner without requiring such mechanical elements as dogs and limit switches.
A characteristic of the aforesaid conventional pitch error correction system is that the pitch error correction factor for a correction along one control axis, such as the correction factor EX.sub.i for the X-axis, depends solely upon the current position of the movable body on said axis and is independent of the current position along the other axes, such as the Y-axis in the above example. In other words, in the foregoing correction system, pitch error correction factors E.sub.x, E.sub.y, E.sub.z for corrections along the respective X, Y and Z axes are expressed as follows: E.sub.x =f.sub.x (x), E.sub.y =f.sub.y (y), E.sub.z =f.sub.z (z). This means that a highly precise positional correction cannot be performed with the conventional system described above in a case where the pitch error correction factor for a certain control axis depends not only upon the current position along said control axis, but upon the current position of the movable body along the other control axes as well.